1. Field of the Invention
The invention relates generally to surface layers with increased roughness, and more particularly to a method for increasing the roughness of a tissue-engaging outer surface of a porous structure without altering the pore size and porosity of the structure, and to medical implants incorporating said porous structure with increased surface roughness.
2. Description of the Related Art
Especially in the medical fields, the surface of an implant, device, or other implement can significantly affect function. For example, attempts have been made to improve bone implant stability by increasing the roughness of the implant. Other attempts have been made to improve bone implant stability by providing pores in the implant for bone ingrowth.
One method of achieving bone ingrowth in implants that contact bone (e.g., orthopedic implants) includes sintering metallic bead surfaces onto a substrate. Other methods of achieving bone ingrowth in implants includes using a reticulated foam porous coating fabricated from titanium that incorporates an electrical discharge machined (EDM) surface treatment, an EDM surface with axial grooves, an EDM surface with cross-hatching, or a photo-etched surface. Foam metal implants have been shown to achieve greater bone ingrowth than sintered bead implants. See, Urban, Robert M. et al., “Biomechanical and Histological Response to a Novel Foam Metal Porous Coating with Comparison of Two Methods for Measuring Bone Ingrowth,” Transactions of the 54th Annual Meeting of the Orthopaedic Research Society, p. 1854, Mar. 2-5, 2008.
However, production of a porous metallic foam ingrowth structure (e.g., one created by applying fine metal powder particles to all surfaces of a porous structure) can require a secondary machining step to obtain the desired shape and dimensions (e.g., tolerances) of the machined metal foam structure. Such machining can cause a loss of roughness on the machined surfaces (e.g., tissue-engaging outer surfaces). The roughness can be maintained or recovered using textured molds during sintering to pressure-sinter particles to a substrate without sacrificing texture for porous bead-coated implants. Alternatively, the roughness for a metallic foam can be maintained or recovered using electrical discharge machining (“EDM”), creating a cross-hatch pattern and, upon implantation, gaps between the grooves in the coating and bone. These mechanisms have thus far proved unsatisfactory in increasing the roughness of machined tissue-engaging outer surfaces of a porous metallic foam ingrowth structure while maintaining the pore size and porosity of the structure.
Therefore, there is a need for an improved method for providing a porous metallic foam structure with improved bone ingrowth characteristics that avoids the drawbacks discussed above.